Mixed light sources are used in many fields nowadays. Thus, by way of example, mixed light sources can be found in fields of image reproduction or projection technology, but also in simple applications such as flashlights. Depending on their use, mixed light sources of different colors are generated in this case.
For use in flashlights, a mixed light source is preferably embodied as a white light source. In this case, the aim is to simultaneously generate and emit as many wavelengths as possible from the visible wavelength range, that is to say wavelengths of from 380 nm to 780 nm. For the operation of a mixed light source by means of energy stores, a very good energy balance is also advantageous in addition to a high color fidelity. This ensures, firstly, that illuminated objects reflect as authentically as possible and familiar subjective perception of the objects is thus obtained. Secondly, a high energy efficiency is advantageous in order to maintain temporally long operation of a mixed light source.
Mixed light sources are often generated by means of semiconductors nowadays and so-called primary colors are generated by simultaneous additive superposition. The primary colors are electromagnetic radiations in a narrow wavelength range, relative to the visible wavelength range, which are generated by an emissive semiconductor chip, for example, LED. By way of example, a generation of wavelengths in the range of 625 nm-740 nm represents the color red, of 520 nm-565 nm the color green, for 450 nm-500 nm the color blue. Other wavelength ranges are likewise conceivable. A simultaneous superposition of these colors in a wide variety of combinations generates mixed lights of different colors.
This superposition of the primary colors has hitherto been carried out by means of additive mixing. In image reproduction or projection technology, for example, subpixels, that is to say pixel subelements, of the colors red, green and blue are driven differently. The colors are superposed differently by means of the driving. Given sufficient distance between the viewer and the image reproduction device or the projection area and by virtue of a high number of such pixels, the impression of a multicolored image arises as a result of additive color mixing.
Furthermore, another possibility is also employed for generating mixed light sources. This usually involves using semiconductor components which emit electromagnetic radiation in a specific narrow wavelength range. This radiation is regularly referred to as primary radiation. In this case, this electromagnetic radiation need not necessarily be completely or partly in the visible wavelength range. This radiation is at least partly converted into a secondary wavelength by means of a luminescence conversion element.
In order to generate the primary radiation, active layers of a pn junction of a semiconductor chip situated in a semiconductor component are doped differently, for example. The resultant energy level differences in the energy level schemes, also referred to as band gaps, lead to an emission of light of different wavelengths. In this case, the wavelength of this emitted light is directly dependent on the energy level difference and adjustable to a limited extent by means of the doping.
The emitted primary wavelength of the semiconductor chip is at least partly converted into a secondary wavelength by means of a luminescence conversion element. For this purpose, it is important for the luminescence conversion element to be introduced into the beam path of the primary radiation of the semiconductor chip in such a way that the totality of the emitted photons of the primary radiation have to cover a path of optically identical length in the luminescence conversion element in order that all the photons to be converted convert to the new secondary wavelength in an identical way.
Semiconductor components which convert the primary wavelength into a secondary wavelength by means of a luminescence conversion element are produced by means of very cost-intensive and complex processes. The aim during production is primarily to introduce the luminescence conversion element into the beam path of the emissive semiconductor chip as far as possible in such a way that the mixed light generated has a wavelength spectrum that is constant over the entire emission range of the semiconductor component. In order to realize this constancy, screen printing and sedimentation processes are customary at the present time. Another method is the spatial separation of the luminescence conversion element from the emissive semiconductor chip.
In the case of production in a screen printing process it is necessary, for example, to ensure that the contact areas of the semiconductor chip are kept free. For this purpose it is necessary to decontaminate the bonding locations. A further problem is the small edge length of the emissive semiconductor chips. Furthermore, with a few micrometers, it is difficult nowadays to position a phosphor directly onto the semiconductor chip. This fabrication variant and the associated necessary cleaning of the fabrication apparatuses are very cost-intensive on account of the complexity.
For the production of a mixed light source by means of a sedimentation process, the phosphor is mixed with a resin and positioned onto the chip. The different viscosities of the two substances are then exploited. On the one hand, the two substances are separated from one another on account of the continuous time by virtue of which the phosphor material sediments from the low-viscosity resin, and a separation of the two substances is thereby accomplished. On the other hand, it is likewise possible to use a thermal process to control the viscosity at which the phosphor sediments starting from a specific threshold temperature.
Both methods make it possible to position a phosphor in such a way that a fluctuation of the wavelength spectrum of the emitted mixed light wavelength with respect to the emission characteristic of the entire semiconductor component is kept as small as possible. What is disadvantageous about these processes, however, is the high outlay for obtaining this constancy.